Download The Greenhouse Effect

The Greenhouse Effect
by Louise Comeau and Tim Grant
Reprinted by OneWorldJourneys.com from “Teaching About
Climate Change,” edited Tim Grant and Gail Littlejohn, New
Society Publishers, 2001, www.GreenTeacher.com.
About half of the radiant energy that comes from the sun
is reflected back into space after striking reflective surfaces
such as particles in the atmosphere or snow at ground level.
The rest is absorbed at the Earth's surface and released as
infrared, or heat, energy. As this heat radiates upwards, some
escapes into space and some is absorbed and emitted back to
Earth by gases, such as water vapor, carbon dioxide, nitrous
oxide and methane, which occur naturally in the atmosphere.
This natural "greenhouse" effect is what makes the Earth's
surface warm enough to support life. Without greenhouse gases,
the average temperature on Earth would be about -180C (0.4ºF).
As long as the energy entering the system is balanced by
the energy leaving, average temperatures remain relatively
stable. But a rise in levels of greenhouse gases has the potential
to upset this equilibrium by increasing the amount of heat that
gets trapped near the Earth's surface. For most of the last 10,000
years, the concentration of the most abundant greenhouse gas,
carbon dioxide, remained fairly steady at 280 parts per million
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(280 molecules of carbon dioxide for every million molecules
of air). During that period, there existed a balance between
sources of carbon dioxide such as respiration, decomposition
and forest fires, and processes that remove it from the
atmosphere such as photosynthesis and absorption by oceans.
Since the Industrial Revolution began in the 1750s, humans
have affected the carbon dioxide balance in two ways. We have
added large quantities of carbon dioxide to the atmosphere by
burning carbon-rich fossil fuels such as coal, oil and natural
gas. At the same time, we have cut down nearly half of the
world's forests,1 releasing the carbon stored in the trees and
reducing the Earth's ability to absorb carbon dioxide through
photosynthesis. In the past decade, almost seven billion tonnes
of carbon were released every year by the burning of fossil fuels
and destruction of forests. About half of it dissolved in the
oceans or was taken up by plants; the other half, about 3.5
billion tonnes of carbon, was added to the atmosphere and will
remain there for 50 to 200 years. So far, the global concentration
of C02 has increased by 31% from pre-industrial levels and is
estimated to be increasing at the rate of 0.4% per year. Methane
levels have increased by 15.1 % and nitrous oxide by 17%. The
most potent of greenhouse gases, a group of human-made
compounds which includes chlorofluorocarbons (CFCs), did
not even exist before the 1930s, but they account for about 12%
of today's enhanced greenhouse effect.2 All of these greenhouse
gases are destroyed over time by atmospheric chemistry, but
the process can take decades to centuries.
MERCURY RISING: Bearing Witness to Climate Change
What can we expect?
Adding more greenhouse gases to the atmosphere than
can be disposed of naturally is like adding an extra blanket to
your bed: things are going to heat up. According to climate
scientists, mean surface temperatures will likely rise between
1.4 and 5.8ºC by 21 00.3 Warming will not be the same the world
over. Because of the way wind and ocean currents transfer
energy from the equator to the poles, and as snow and ice melt,
the region north of 500 N is expected to warm more than the
global average. Summer temperatures could increase by as
much as 70C in Canada's Mackenzie Basin, with increases of
up to 90C in April and May.4
Because heat is what drives the climate system, we can
expect an increase in extreme weather. Among the many impacts
of climate change that scientists now predict are: a rise in sea
level and changes in currents; changes in precipitation patterns;
shifts of temperature zones; an increase in the frequency and
intensity of storms; more heat waves and droughts; and a higher
incidence of forest fires.
We are already seeing these effects in North America. Karl
et al reported in the February 1996
Bulletin of the American
Meteorological Society that there is
less than a one in 1,000 chance that
the observed increase in extreme
one-day precipitation events in the
United States could be a naturally
occurring event. In the north, the
Mackenzie Basin has warmed by
an average of 1.5ºC over the last
100 years,5 more than double the
global rate (0.6ºC). Alaska and
eastern Siberia have warmed at
similar rates.
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The Greenhouse Effect
What can be done?
To slow the pace of climate change, we will have to reduce
overall greenhouse gas emissions from many sources, but
especially carbon emissions associated with fossil fuel
consumption and the clearing of tropical rainforests. In North
America, most analysts agree that we need to eliminate subsidies
to the oil, gas and coal industries while promoting energy
efficiency, renewable energy technologies, and the development
of transportation systems that allow people to leave their cars
at home. Many argue for "carbon taxes" on the use of fossil fuels
that would encourage more resource-efficient activities. Much
research has shown that taking steps to reduce carbon emissions
actually enhances economic development. In a 1996 study, the
Sierra Club of Canada found that stabilizing and reducing
greenhouse gas emissions in Canada could create up to 1.5
million person-years of work, far in excess of the job creation
potential of the current oil sands expansion in northern Alberta.
To reduce carbon dioxide emissions in the short term, the
single most effective measure, and the least disruptive
economically, would be to raise the fuel efficiency standards of
new cars and trucks which have remained unchanged for almost
a decade. On an individual level, planting trees, favoring public
Pathways of solar radiation:
1. Light strikes particles in the atmosphere and is reflected
back to space.
2. Light is absorbed at the surface and released as heat;
then absorbed by clouds and emitted back to Earth.
3. Light strikes water and is reflected back to space.
4. Light is absorbed at the surface and released as heat;
then absorbed by greenhouse gases and emitted back
to Earth.
5. Light is absorbed at the surface and released as heat;
the heat escapes directly into space.
MERCURY RISING: Bearing Witness to Climate Change
The Greenhouse Effect
transit and bicycles over cars, making our homes more energy
efficient, reusing and recycling materials, and eating locallygrown foods are actions that everyone can take to help reduce
emissions of greenhouse gases.
Louise Comeau is the Director of the Sustainable Communities
Department of the Federation of Canadian Municipalities in
Ottawa, Ontario. Tim Grant ('What Can Be Done?") is co-editor
of Green Teacher magazine.
Notes
1 Lester R. Brown, et al., State of the World 1999 (Washington,
DC: Worldwatch Institute, 1999), p. 60.
2 Intergovernmental Panel on Climate Change (IPPC), Climate
Change 2001: The Scientific Basis, Summary for Policymakers,
World Meteorological Organization/ United Nations
Environmental Programme, p. 7.
3 IPCC, p. 13.
4 Janet Brotton and Geoffrey Wall, "The Possible Effect of
Climate Change on the Sport Hunting of Bathurst Caribou
of the NWT," Mackenzie Basin Impact Study, Interim Report
#2, 1994.
5 Environment Canada, The State of Canada's Climate:
Monitoring Variability and Change, 1995.
THE MAJOR GREENHOUSE GASES
Gas
Contribution
to the
greenhouse
effect
Increase
since 1750
Heat-trapping
ability
(in relation
to C02)
Lifespan in
atmosphere
(years)
Carbon
dioxide
(C02)
53%
31%
1
50-200
Respiration; decomposition;
forest fires; evaporation from
oceans; burning of fossil fuels.
Methane
(CH4)
17%
151%
25
10
Underground deposits (natural
gas is mostly methane);
respiration by anaerobic
decomposers living in wetlands,
rice paddies and the digestive
tracts of ruminant animals and
termites; garbage dumps.
Nitrous
oxide
(N20)
5%
17%
200
150
Microbes that break
downorganic fertilizers; burning
of fossil fuels and wood.
Ground-level
ozone
(03)
13%
36%
2,000
weeks
Very small amounts naturally
present in atmosphere; formed
photochemically when nitrogen
oxides and volatile organic
compounds in automobile
exhaust react in sunlight.
Halocarbons
(CFCs)
12%
none in 1750
up to 10,000
60-100
Human-made compounds used
in refrigerators, air conditioners,
foam products, aerosol sprays.
There are no natural sources.
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Sources